1. Field of the Invention
This invention relates to polarization control such as for an optical heterodyne or homodyne receiver wherein an optical waveguide WL1 receives at an input e.sub.l for coupling an elliptically polarized optical wave Tln transmitted by a subscriber and includes a phase modulator PM1 mounted to a first waveguide WL1 so as to continuously vary the preparation conditions of the signal in the first waveguide WL1 and a polarization converter PK1 follows the first phase modulator PM1 and another optical waveguide WL2 which receives an input signal e.sub.2 for coupling a linearly polarized optical wave supplied by a local oscillator LO and another polarization converter PK2 and another phase modulator PM2 following the second polarization converter PK2 so as to vary the phase in the second waveguide WL2 and to yield the same states of polarization of either optical waves.
2. Description of the Prior Art
German Patent Application No. P 36 10 573.2 (VPA No. 86 P 8019 DE) and an article by Reinhald Noe entitled "Endless Polarization Control For Heterodyne/Homodyne Receiver", Fiber Optic 86, London, April/May 1986 disclose five birefringent elements comprising three-phase modulators and two polarization converters which are alternately arranged along an optical waveguide. Every polarization condition at the input side can be converted without restriction to any arbitrarily desired polarization condition with such arrangement. "Unrestricted" means that a continuous ongoing polarization adaptation to an arbitrarily desired or prescribed polarization condition is assured without a functional polarization resetting when limits of the control variable are reached.
There is a problem of matching the state of polarization ("SOP") in optical heterodyne or homodyne receivers. Following in the specification the abbreviation "SOP" is used for "state of polarization". The polarization of the optical signal supplied to the receiver must be matched to the SOP of the optical signal of a local oscillator of the receiver. The signal of the local oscillator is generally linearly polarized, but the signal supplied to the receiver can assume any arbitrary SOP. So as to bring the SOPs of both signals into agreement, either the SOP of the signal supplied to the receiver must be transformed into the same linear polarization condition of the signal of the local oscillator in a polarization stabilization stage and then be kept constant in time or the SOP of the signal of the local oscillator is to be changed to the SOP of the signal supplied to the receiver which can continuously change. These two alternative solutions can be accomplished with an arrangement discussed by the article by Noe listed above.
In the first alternative, the SOP at the output of the arrangement is fixed whereas the polarization in the second alternative arrangement is fixed at the input of the arrangement. In such arrangements according to Noe, a phase modulator can be omitted so that only four birefringent elements are required.
The embodiment of the arrangement discussed in the article by Noe is a fiber optical realization wherein the birefringences are generated by exerting an external squeeze onto the waveguiding fibers which form the waveguides. Since it requires only two different birefringent elements, it should according to the article by Noe facilitate an integrated optical structure.
The earlier German Patent Application No. P 36 15 982.4 (VPA No. 86 P 8027 DE) proposes a fiber optical arrangement comprising only three birefringent elements, two phase modulators and a polarization converter which is controllable such that no abrupt change of the polarization results even when any of the elements has reached a limit of its control range. An integrated optical arrangement which is similar to the apparatus described in German Patent Application No. P 36 15 982.4 is described in an article by Alferness, R. C. entitled "Electro-Optic Guided-Wave Device For General Polarization Transformation" IEEE Journal Quant. Electr. QE-17 (1981), Pages 965-969. In this arrangement, the three existing elements, two electro-optically induced, birefringent phase modulators and a polarization converter are arranged in an alternate form along an optical waveguide integrated in a substrate of electro-optical material. In the specific embodiment of this arrangement described therein, the optical waveguide is composed of a Ti diffused lithium niobate waveguide extending in the y-direction which proceeds at the surface of a x cut lithium niobate crystal. Every phase modulator is realized by an electrode pair for applying a control voltage arranged on both sides of the waveguide and applied to the surface of the crystal. The polarization converter is composed of a series of electrodes, arranged transversely over the waveguide for generating vertical electrical fields in the waveguide which periodically change along the waveguide.
An arbitrary SOP can also be converted into another arbitrary SOP with such an arrangement. This system is suitable for stabilization of the SOP of a signal received from a fiber particularly for the stabilization of an elliptically polarized signal typically received from a monomode fiber into a linearly polarized signal.
With this prior art arrangement, an unrestricted continuous polarization matching assuming ongoing polarization drift is not possible and resetting is required when a certain limit of the control range has occurred. The polarization converter utilized in the specific embodiment described in the article by Alferness and the entire embodiment has an optical bandwidth of about 2 nm and are therefore unsuitable for use in an optical communication transmission system and, thus, are also unsuitable in an optical heterodyne or homodyne receiver See also the article by Booth, R. C., Daymont-John B. E., Sturges P. E., Wilson, M. G. entitled "Temperature Curing of LiNbO.sub.3 Electro-Optic Waveguide TE/TM Mode Converters", Electron. Lett. 20 (1984), Pages 1045-1047.
For such use, such an arrangement must be optically broadband that it fulfills the function for the desired wavelength region, for example, 1300.+-.25 nm and no temperature stabilization and electrical wavelength region corrections are necessary.
An article by Thaniyavarn, S., entitled "Wavelength Independent, Optical Damage-Immune Z-Propagation LiNbO.sub.3 Waveguide Polarization Converter", in Application Phys. Letter 47 (1985) pages 674-677 and the article by Thaniyavarn, S., entitled "Wavelength Independent, Optical Damage-Immune LiNbO.sub.3 TE-TM Mode Converter", Optics lett. 11 (1986), Pages 39-41, disclose a broadband electro-optically induced, birefringent polarization converter integrated on a substrate of electro-optical material wherein a TI-defused lithium niobate waveguide is arranged on the surface of a x-cut lithium niobate crystal in the z-direction and wherein three electrodes extending in the longitudinal direction of the waveguide i.e. in z-direction are applied on the surface of the crystal. Two of these electrodes are arranged on either side of the waveguide and the third electrode is arranged between the other two ones above the waveguide. By applying three suitable voltages to these electrodes, an electro-optically induced matching of the effective refractive indices for TE-polarized and TM-polarized optical waves can be achieved which allows a nearly 100% conversion. The article by Mariller, C., Papuchon, M. entitled "A Simple and Wide Optical Bandwidth TE/TM Converter Using Z Propagating LiNbO.sub.3 Waveguides", Proc. 3rd European Conference, ECIO 1985 Berlin, Springer Series in Optical Sciences 48 (H.-P. Nolting, R. Ulrich Editors), Pages 174-176 discloses an integrated optically broadband polarization converter which differs from the converter of Thaniyavarn only in that it utilizes a y-cut crystal which requires a horizontal field transversely to the waveguide for the TE/TM conversion and wherein the third electrode over the waveguide is missing and therefore the degree of freedom with respect to the optically induced matching of the effective refractive indices is lacking.